Fill in the blank with the most appropriate word.
The war ________ in 1939.

In the given sentence "The war started in 1939," the verb "started" is in its past simple tense form. The sentence talks about an event (the war) that occurred in the past at a specific time, which is 1939. The past simple tense is used to describe completed actions or events that happened at a definite time in the past.

The other options are not suitable for this sentence. Option A, "start," is in the present tense, which does not fit the context of the sentence referring to a past event. Option B, "will start," is in the future tense, which also does not match the timeline of the sentence. Option C, "is starting," is in the present continuous tense, which is not appropriate for describing a completed event in the past. Therefore, the correct answer is "started" (Option D).